When receiving radio waves of a mobile phone, a wireless LAN, terrestrial digital broadcasting, or the like, the reception performance is degraded due to the effects of the arriving waves (hereinafter called “delayed waves”), which arrive after reflection or scattering on buildings, vehicles, or the like, in addition to the arriving wave (hereinafter called “a direct wave”) which arrives directly from the transmitter. An environment in which a plurality of arriving waves are present is called a multipath environment.
A known technique to reduce the degradation in the performance due to multipath is a directivity control using an array antenna. An array antenna has a plurality of antenna elements, and can be made to have a directivity by controlling weighting coefficients used for combining signals received by the antenna elements. In a multipath environment, the degradation in the performance due to the influence of the delayed waves can be mitigated by controlling the directivity so that the main lobe is directed to the direction in which the direct wave arrives. In order to improve the reception performance by the directivity control of the array antenna, it is necessary to accurately estimate the direction in which the direct wave arrives.
If the receiver is fixed, and the direction of the transmitter is known in advance, the directivity may be manually adjusted so that the main lobe of the array antenna is directed to the transmitter. When the radio waves are received at a moving body such as a vehicle, for instance, in a case of reception in an inter-vehicle communication, manual adjustment is not feasible because the position of the transmitter relative to the receiving apparatus changes with the movement of the vehicle. It is therefore necessary to automatically estimate the arrival direction of the direct wave, from the received signal in which the direct wave and the delayed waves are multiplexed.
Radio wave environments for the wireless communications can be classified into LOS (Line Of Sight) in which the transmitter is in a visual line of sight from the receiver, and NLOS (None Line Of Sight) in which there is no visual line of sight between the transmitter and the receiver. The present invention assumes the LOS environment.
Patent Reference 1 describes an apparatus in which the multipath arrival directions are measured based on signals received by two antennas. In this apparatus, the frequency characteristic (transfer function in the frequency domain) of the transmission channel is estimated from the signal received by each antenna element, the estimated transmission channel frequency characteristic is inverse-Fourier transformed to determine a complex delay profile, the arriving waves of different delay times are separated from the complex delay profile, and the arrival angle is estimated on the basis of the phase difference between the separated direct waves received by the antenna elements.
Patent Reference 2 discloses estimation of the delay times by a super-resolution process, such as an MUSIC (MUltiple SIgnal Classification) process, or an ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) process. According to the technique described in Patent Reference 2, the signals received by a plurality of antennas are transformed into a frequency spectrum, the delay time of each arriving wave is estimated by a super-resolution process using the frequency spectrum, the estimation results are used to estimate a coefficient matrix in which the arriving waves are included, the above-mentioned frequency spectrum is multiplied by a pseudo-inverse matrix of the above-mentioned coefficient matrix to separate the components of the direct waves, and the arrival angle is estimated from the phase differences among the separated direct waves.